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Science in Dante’s “Paradiso”

In his Paradiso, which completes the trilogy begun with the Inferno, Dante travels through the (Ptolemaic) heavens, which look like this:

While most of Dante’s astronomy has been rendered obsolete by discoveries that began with Copernicus, Dante did understand the nature of the Milky Way. The Paradiso, it is true, expresses a degree of doubt regarding this:

“As, graced with lesser and with larger lights
between the poles of the world, the Galaxy
gleams so that even sages are perplexed;” — Paradiso, XIV, 97–99, tr. Mandelbaum

“In the Old Translation [Aristotle] says that the Galaxy is nothing but a multitude of fixed stars in that region, so small that we are unable to distinguish them from here below, though from them originates the appearance of that brightness which we call the Galaxy … Avicenna and Ptolemy seem to share this opinion with Aristotle.” — Convivio, II, 14, tr. Lansing

Milky Way; photo by “Eclipse.sx”

In the Paradiso, Dante discusses more than just theology and astronomy. He somehow manages to work in Thales’ theorem, for example (Paradiso, XIII, 101–102). What’s more, having told us in the Purgatorio (XV, 16–21) that “the angle of incidence is equal to the angle of reflection,” Dante now proposes an actual experiment in optics (no, it wasn’t Galileo who invented the experimental method!):

“Yet an experiment, were you to try it,
could free you from your cavil – and the source
of your arts’ course springs from experiment
Taking three mirrors, place a pair of them
at equal distance from you; set the third
midway between those two, but farther back.
Then, turning toward them, at your back have placed
a light that kindles those three mirrors and
returns to you, reflected by them all.
Although the image in the farthest glass
will be of lesser size, there you will see
that it must match the brightness of the rest.” — Paradiso, II, 94–105, tr. Mandelbaum

The image above (click to zoom) is the result of replicating Dante’s proposed experiment with the Persistence of Vision Raytracer. The unnecessary third mirror tells us that Dante is here also speaking allegorically about the reflection of Divine light, and that – hinting at 1 Corinthians 13:12 – he is looking forward to his final vision of the Trinity, in what is after all a theological poem:

“That light supreme, within its fathomless
Clear substance, showed to me three spheres, which bare
Three hues distinct, and occupied one space;
The first mirrored the next, as though it were
Rainbow from rainbow, and the third seemed flame
Breathed equally from each of the first pair.” — Paradiso, XXXIII, 115–120, tr. Sayers